This invention relates to devices for selecting from a plurality of air sources, air which would be most efficient to condition to a predetermined temperature. In particular, it relates to pneumatic devices which select air from two sources on the basis of humidity and temperature of air from each source.
In buildings which have a central air conditioning system, air is generally supplied to the air conditioning system from two sources, namely, from the outdoors and from the enclosed space being conditioned. The regulations and building codes in many localities require that a certain percentage (e.g. 20%) of air supplied to the enclosed space be outdoor air. The remaining air supplied to the enclosed space can be any proportion of the outdoor air and the return air from the enclosed space. To save energy, control systems have been designed, to choose either outdoor or return air depending on which air requires less energy to bring it to the desired temperature. Initially, such control systems operated solely on the basis of temperature of the outdoor and of the return air. For example, when the desired temperature in the enclosed space is 72.degree. F., the outdoor air temperature is 73.degree. F., and the return air temperature is 77.degree. F., the control system selects practically all outdoor air and excludes the return air. It is known, however, that the amount of energy required to cool a given volume of air depends not only on its temperature but also on its moisture level.
Accordingly, humidity as well as temperature of the air from each source should be considered in selecting between outdoor air and return air. The air having a lower enthalpy should be chosen in all but one situation. When the outdoor air has an enthalpy lower than that of the return air but the temperature of the outdoor air is higher than that of the return air, it has been determined that the return air takes less energy to condition. Accordingly, an efficient control system should select the source of air for the air conditioning on the basis of the enthalpy and temperature. In the event the enthalpy of outdoor air is lower but its temperature is higher than that of the return air, the control system should choose the source having a lower temperature i.e. the return air. The present invention provides an improved control device (hereinafter referred to as "a comparator") for selecting of air from two sources of the basis of humidity and temperature of each of two air sources. When the selection is made on the basis of enthalpy, the comparator of the present invention does not generate enthalpy signals but rather generates a force which is indicative of the difference in relative humidity levels of air from each of the two sources and another force indicative of the difference in dry-bulb temperatures of air from each of the air sources and on the basis of these comparisons selects the air which has a lower enthalpy. When the selection is made on the basis of enthalpy, the humidity and the temperature comparison sections coact to give the ultimate selection signal. On the other hand, when the selection is made on the basis of temperature, the humidity comparing section does not coact with the temperature comparing section.
Thus, one object of the present invention is to provide an improved pneumatic comparator which provides a signal indicating which one of the two sources of air would require less energy for cooling it to a desired temperature.
Another object of the invention is to provide a comparator which produces, without generating intermediate pneumatic signals indicative of enthalpy of air from each source, a pneumatic output signal indicating which of two air sources requires less energy for conditioning it to a desired temperature.
Still another object of the invention is to provide an improved pneumatic comparator which, in response to pressure signals indicative of the dry-bulb temperature and the relative humidity of air from each of the two sources, generates a digital signal indicating which air source requires less energy for cooling to the desired temperature.
A further object of the present invention is to provide an improved pneumatic comparator which is simple and inexpensive to construct, install and maintain.
A still further object of the present invention is to provide an improved pneumatic comparator which is compact and reliable.
Still another object of the present invention is to provide an improved pneumatic comparator which can be easily and quickly modified to operate with either the two-pipe pneumatic transmitters or the single-pipe pneumatic transmitters or with both single-pipe and two-pipe transmitters.
A still further object of the present invention is to provide an improved pneumatic comparator which includes a section for generating a force indicative of the difference in dry-bulb temperatures and a section for generating a force indicative of the difference in humidities of each of the two air sources. The two sections can either coact or can function independently of each other. When the two sections coact the output signal indicates which of the two air sources has a greater enthalpy. On the other hand, when the two sections do not coact, the output signal indicates which of the two air sources has a higher temperature. Accordingly, the same temperature section is used in both modes of operation.
Other objects of the present invention will become apparent to those skilled in the art upon studying this disclosure.